1. Field
The present application relates to optical metrology, and more particularly to optical metrology of structures formed on semiconductor wafers using optical pulses.
2. Related Art
Optical metrology involves directing an incident beam at a structure, measuring the resulting diffracted beam, and analyzing the diffracted beam to determine a feature of the structure. In semiconductor manufacturing, optical metrology is typically used for quality assurance. For example, after fabricating a periodic grating in proximity to a semiconductor chip on a semiconductor wafer, an optical metrology system is used to determine the profile of the periodic grating. By determining the profile of the periodic grating, the quality of the fabrication process utilized to form the periodic grating, and by extension the semiconductor chip proximate the periodic grating, can be evaluated.
In conventional optical metrology, the diffraction response versus the light wavelength is typically measured. The spectral decomposition of the light can be done either in the incident light path or in the reflected light path. To this end, a monochromator is typically used, which is based on the light dispersion by a prism or a grating. The dispersed light then can be directed to a detector-array such as a charge coupled device (CCD) array, or it can be moved consecutively swept over one detector. In the first case, the accuracy is limited due to the tolerances throughout the detector elements. In the second case, the measurement time is increased due to the motion.